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Darwin Correspondence Project

From D. B. Miller   12 May 1877

Cincinnati Ohio U.S.A.

May 12th 1877

Chas. Darwin Esqr. FR.S. etc | Down Beckenham Kent England

Dear Sir

I have been reading your very interesting work on Sexual Selection.1 I did not bring to its perusal by any means a scientific knowledge—only that amount that was acquired by studies necessary to prepare myself as a Physician and even these studies have been long interrupted by business pursuits

While reading your work I could not help thinking that you do not give sufficient prominence to a very important factor or at least one that seems to me to be a starting point. There is an essential difference in the reproductive functions of the male and female. How this difference arose in the progress of evolution you may be able to explain, but I start with the idea of this differentiation. In the male the stimulus arising from its specialized reproductive organs is a constant quantity liable to be sure to some variation in intensity, as for instance during the breeding season. In the female however this stimulus is an interrupted, recurrent quantity. It seems to me, the influence of the testicle is more potent and persistent than that of the ovary. The stimulus from the testicle is stronger and ever present—from the ovary it is weaker periodical with partially dormant intervals. If this is so the male would necessarily be different from the female in physical and moral characteristics   It is this stronger force in the system that makes the male more pugnacious than the female as a rule. You have given some curious instances in birds when the female shows the most pugnacity and in these cases the females are the more highly colored as to plumage—the instincts of the two being completely transposed,2 I think even in this case the starting point of the difference is the fact that in this particular class, the stimulus from the reproductive organs is exceptionally stronger in the female than in the male, just as we sometimes see some women with very strong sexual passions and my observation is that they are usually masculine in their characteristics. Anyhow in the instance you give the female sought and fought for the male indicating more ardour and had some other characteristics of the male as brighter plumage. The conclusion is as the more ardent passion gave it the moral characteristics of the male it also gave it the physical characteristics that usually belong to the male   Where gelding is practised there is always a modification of the physical characteristics as well as the pscychical. I do not know that spaying has ever been practised except in the son and the change effected by it in the physical appearance is not appreciable according to my observation But the main fact to which I call your attention is that the stimulus of the reproductive organs is a modifying constitutional influence and usually this has greater force in the male than in the female. You say yourself in the breeding season when this stimulus is strongest, the comb, wattles and plumage of the males feel the influence and exhibit it by heightened color etc. you constantly speak of the greater eagerness and ardour of the male.3 This shows a strong stimulus in the constitution that certainly would modify to a much greater extent than any slight changes in the conditions of life. As a matter of course this view does not affect your arguments as to what is accomplished by variability sexual selection and the complex laws of inheritance. You merely have in the male and female a constitutional difference that of itself will modify which can be modified still further by variability sexual selection and inheritance. Emasculation of the male effects considerable changes in physical characteristics, even in some instances to the extent of changing the color. It arrests the growth of appendages that you consider ornamental   Does not this show that the stimulus of the testicle has an important influence on physical characteristics. It leads my mind to the conclusion that the male has various differences in physical conformation because he is a male and therefore subject to a stimulus adequate to produce these differences. I think whatever these may be we can safely conclude that they would be limited in transmission to the male. Whatever marks come from variability, not dependent upon the influence of the reproductive organs would be transmitted to both sexes and I think varieties have been formed merely on account of some variation being coincident with a favored male who through success in securing females became the father of a numerous progeny and it would be easy for this variation to become cumulative through interbreeding. I know within my own observation that a large majority of the equine race in a neighborhood where I was raised, became mouse colored from the fact that the only stallion that served the mares for many years in this district was mouse colored. Suppose now in a state of Nature this stallion beat all rivals it would have produced a mouse colored variety.

In closing I will merely say that these views may be entirely the result of a limited vision for I confess the only knowledge I have on the subject comes from reading your works, for pastime rather than study.

Yours very Respectfully | D. B. Miller M.D.

P.O. Box #271 Cincinnati Ohio U.S.A.


For CD’s examples, see Descent 2: 200–8.
In Descent 1: 271–9, CD compared differences between males and females in animals and humans, arguing that the adult male was nearly always more modified from the juvenile than the female, and that males possessed stronger passions. The modifications were used mostly for combat and display in the pursuit of females.


Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.


Offers a supplementary component to sexual selection which CD ignored in Descent: the direct effect of differences in ardour between males and females.

Letter details

Letter no.
David Benton Miller
Charles Robert Darwin
Sent from
Source of text
DAR 171: 179
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 10959,” accessed on 23 September 2021,